When you say "rendered," do you mean hitting the Enter/Return key to produce Rendered files, for smoothest playback, or do you mean once you Export/Share/Publish the Timeline to an output file, the black bars show up?
What version of PrE are you using?
Also, can you give us information on the Video Clips in your Project, such as the camcorder that they came from, the CODEC used, and their Frame Size?
Next, what Project Preset did you choose at New Project?
If you did mean after you have output the Timeline to a file, what were the specs. that you chose for that output file?
Black bars in the output file, or on the Timeline, indicate a mis-match somewhere in the process.